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Picking the Perfect Pump
When drawing up purchase specifications for pumpers, a fire department must choose the type of pumping equipment to be used. Although centrifugal fire pumps are available today, there are two basic types from which to choose: the single-stage and the two-stage series/parallel pump.
Three main criteria are used in making this decision: performance, simplicity of operation and pump life (the first cost differential is not a significant amount when compared to the overall cost of the complete apparatus over its useful life.)
During the past several years, the average rated capacity of new pumpers has increased so that, today, 1500-gpm to 2000-gpm pumpers are more common, than the 500-gpm thru the 1250-gpm pumpers. Single- stage pumps also have been fairly popular; however, two-stage series/parallel pumps, which have important advantages over single-stage regardless of the capacity rating, are overwhelmingly superior when the rating is 1500 gpm or more. The single-stage pump has a performance advantage only when pumping near rated capacity. Under any other condition, the two-stage pump performs better.
The truth is that a single-stage pump is more efficient than a two-stage series/parallel pump when operating at rated capacity (gpm) and pressure (PSI) and that a two-stage pump can develop higher pressures than a single-stage pump. If a fire department pumper were always operated at one capacity and pressure, and accordingly at a constant speed, a single-stage unit would be preferable as long as the speed necessary could be matched to the driver speed.
That is why almost all stationary pumps are single- stage units, operating at driver speed (direct-connected), unless the pressure to be developed requires two or more stages.
But fire department pumpers don't operate at the same capacity and pressure all the time. In fact, they most commonly are used to pump only a fraction of their rated capacity, usually at fairly low pressure, but often at pressures much higher than the 150 PSI at which the rated capacity can be efficiently pumped.
The performance comparison can best be appreciated by calculating performance for each type of pump, with a particular engine. Performance is affected by the pump transmission ratio selection and the impeller design used, but if pumps having similar impeller designs are compared, the results will be quite reliable.
So, lets compare a single-stage pump and a two stage series/parallel pump, each rated 1500 gpm, the former having a single double-intake impeller (basically, two single-intake impellers mounted back-to-back), as illustrated in Figure 1, and the latter, two single-intake impellers separated by an inter-stage seal, as illustrated in Figure 2.
The two-stage is driven by a Cummins ISC engine rated 330 horsepower at 2000 rpm. For capacities of more than 800 gpm, the two-stage pump usually must be operated in parallel, so that its power requirements are higher than those of the single- stage unit, as shown by the data for 1050 gpm at 200 PSI; however, at lesser capacities the two-stage pump can be used in series and power requirements will always be less than those of the single-stage pump.
The single-stage unit must be able to pump 750 gpm at 250 PSI, at an engine speed below the full-load governed speed of 2200 rpm. As the required impeller speed is 4100 rpm, a 2.27 ratio will provide the required impeller speed at 1806 rpm engine speed, but then the engine speed will be only 1533 rpm for rated capacity at 150 PSI. On the other hand, the two-stage pump can pump 750 gpm at 250 PSI, when placed in series ("pressure") mode, at an impeller speed of only 3300 rpm, and the same 2.27 ratio will provide the impeller speed of 3780 rpm needed to pump 1050 gpm at 200 PSI at an engine speed of 1665 rpm, and that of 3470 rpm needed for rated capacity at 150 PSI the engine speed of 1529 rpm.
The two-stage pump, in addition, has the potential for developing pressures well over 400 PSI if the engine is operated at full-load governed speed. Now, the two- stage pump requires 193 hp for rated capacity at 150 PSI, while the more efficient (at full capacity) single- stage pump needs only 184 hp. The two-stage pump has better over-all performance than the single-stage: comparable capacity at working pressures below 200 PSI; superior performance at pressures from 200 to 300 PSI; plus a high pressure capability of more than 400 PSI compared to less than 300 PSI with the single-stage unit.
Consider, too, that, even though net pressures of more than 250 PSI may be unnecessary under existing conditions, they may be needed in the future. And, of course, it may be desirable to have the capability to develop such pressures for occasional long relays, using high pressure hose when needed. The proliferation of high- rise buildings in suburban ares as well as in larger cities may present an occasional need for high pressures, particularly when the fixed fire protection system fire pump(s) fail for any reason, and it is necessary to deliver water through the standpipes with an effective nozzle pressure at the hose connections at the upper floors. (New York City pumpers purchased during the past several years have been required to pump 250 gpm at 600 PSI, and a number of new pumpers currently being procured must deliver 500 gpm at 700 PSIg.)
The single-stage pump with an added booster stage is capable of developing high pressures at low capacities, but the efficiency of the basic single-stage pump will be low at these capacities, so that it has performance advantages over the single-stage pump only if pressures above those attainable with the basic pump are desired, and none over the two-stage pump.
Simplicity of Operation
The single-stage pump is simpler to operate than the two-stage series/parallel pump because the operator of the latter must decide whether to place the pump in "volume" or "pressure," while no such decision is necessary with a single-stage pump. In actuality, however, the decision need be made only rarely, as the series/parallel pump can be kept in "pressure" mode (series) and changed only if the flow rate (capacity) demanded by the officer in charge cannot be obtained in "pressure." As the vast majority of working fires do not require flow rates of more than 70 percent of rated capacity, the change will not have to be made while pumping at a fire scene, except rarely.
This does not mean that the transfer ("change-over") valve shouldn't be operated at other times. The best way to assure that it will be operational if needed is to operate it daily or at the start of each shift. On the other hand, if a pumper is likely to be used more often at high flow rates, the pump can be kept in "volume" (parallel) where its characteristics will be similar to those of a single- stage pump, which, essentially, it is when in this operational mode.
The two-stage series/parallel pump having both impellers on a common shaft, which is the commonly used type, has two impellers and an interstage seal, compared to the single-stage pump's single impeller. The latter, however, in a pump of 1000 gpm or larger capacity, is a double suction type, all that it is essentially two single-suction impellers, such as those in the two- stage pump, mounted back-to-back. One would expect to have about the same wear occur on the impeller hubs and wear rings in both types of pumps, plus some wear on the interstage seal on the two-stage unit. Then, of course, the transfer valve used in the two-stage series/ parallel pump may have to be repaired or replaced occasionally, along with impellers and wear rings, particularly if it is used with water containing unstrained sand and grit (which are often found in municipal water mains and not merely in water pumped from ponds or small streams). Handling dirty water at high speed and low capacity is a greater threat to impeller life than hub wear.
At capacities significantly below the rated capacity, a centrifugal pump is being seriously misused. As stated previously, the efficiency will be low, and noise and vibration often will occur because the water must flow into, through, and out of the impeller in a much different way than that for which the impeller was designed. This effect is accentuated if the impeller speed is high and if sufficient water does not flow to keep the water and the impeller cool. This kind of operation can result in severe damage to an impeller, including cavitation erosion in the impeller eye and along the vanes, and even, in severe cases, catastrophic failure.
Obviously, a single-stage pump is much more likely to be subjected to this sort of use than a two-stage pump. When operated at capacities of less than 70 percent of rated capacity, the two-stage unit is operating in "pressure." Consequently, the percentage of rated capacity for the series/parallel pump will be twice as high, the speed lower, and the power consumption much lower than for the single-stage.
If the pump is operated at shut-off (zero capacity) for any length of time, the heat buildup in a single-stage unit can become quite dangerous. For example, with an adequate cooling line in use to circulate water from the tank through the pump, the pump would develop 200 PSI with 100 gallons of water in the tank. The temperature of the water would increase from 70°F to 212°F in less than 30 minutes with the single-stage pump, but the same rise would take more than an hour in the two-stage unit. (One horsepower equals 42.4 Btus/minute, and one Btu will heat one pound of water at the rate of 1°F/minute. Therefore, each horsepower will heat five gallons of water (42 lb.) about one degree per minute.) But, if no cooling line were in use, with only 15 gallons of water in the pump to absorb the heat, the single-stage pump's water temperature would increase at a rate of 35°F or more per minute and the two-stage pump's at only about 15°F per minute.
Two-stage series/parallel pumps are preferred over single-stage pumps for the following reasons:
- Pumper performance is better with a two-stage pump over almost the entire operational range, and particularly at capacities less than 70 percent of rated capacity and pressures greater than 150 PSI.
- Pumpers equipped with two-stage pumps can easily develop pressures of 400 PSI and more if these are needed, while those having single-stage pumps are usually limited to about 300 PSI.
- The additional operator training required for pumpers having two-stage series/parallel pumps is minimal.
- Pump life is greater for two-stage units, particularly if pumpers are used often for low capacity/high pressure applications.
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